Encipher.it - Encrypting email with Encipher.it is a simple way to protect your sensitive data.
Horde - Horde Groupware is a free, enterprise ready, browser based collaboration suite.
ProtonMail - Secure email with absolutely no compromises. Get your free encrypted email account today.
Roundcube - Web-based IMAP email client
Temp.PM - Temp.PM - Temporary Private Message
Mailo - Mailo is an email client where you can send and receive emails to and from anyone with an email address.